U.S. patent number 8,723,512 [Application Number 13/685,287] was granted by the patent office on 2014-05-13 for circuits and methods for generating a threshold signal used in a magnetic field sensor based on a peak signal associated with a prior cycle of a magnetic field signal.
This patent grant is currently assigned to Allegro Microsystems, LLC. The grantee listed for this patent is James M. Bailey, Eric Burdette, Daniel S. Dwyer, Jeff Eagen, Glenn A. Forrest, Christine Graham, P. Karl Scheller, Eric Shoemaker. Invention is credited to James M. Bailey, Eric Burdette, Daniel S. Dwyer, Jeff Eagen, Glenn A. Forrest, Christine Graham, P. Karl Scheller, Eric Shoemaker.
United States Patent |
8,723,512 |
Burdette , et al. |
May 13, 2014 |
**Please see images for:
( Certificate of Correction ) ** |
Circuits and methods for generating a threshold signal used in a
magnetic field sensor based on a peak signal associated with a
prior cycle of a magnetic field signal
Abstract
A circuit to detect a movement of an object includes a magnetic
field sensing element for generating a magnetic field signal
proportional to a magnetic field associated with the object and a
motion detector to generate a motion signal indicative of the
movement of the object. The motion detector includes a peak
identifying circuit to provide a peak signal and a peak sample
selection module that selects a sample associated with one or more
prior cycles of a magnetic field signal to generate a selected peak
signal. The motion detector further includes a threshold generator
to generate a threshold signal as a function of the selected peak
signal and a comparator to compare the threshold signal with the
magnetic field signal to generate the motion signal. Peak samples
from prior magnetic field signal cycles may be averaged for use to
establish the threshold signal. A method associated with the
circuit is also described.
Inventors: |
Burdette; Eric (Newmarket,
NH), Bailey; James M. (Concord, NH), Dwyer; Daniel S.
(Auburn, NH), Eagen; Jeff (Manchester, NH), Forrest;
Glenn A. (Bow, NH), Graham; Christine (Bow, NH),
Shoemaker; Eric (Pembroke, NH), Scheller; P. Karl (Bow,
NH) |
Applicant: |
Name |
City |
State |
Country |
Type |
Burdette; Eric
Bailey; James M.
Dwyer; Daniel S.
Eagen; Jeff
Forrest; Glenn A.
Graham; Christine
Shoemaker; Eric
Scheller; P. Karl |
Newmarket
Concord
Auburn
Manchester
Bow
Bow
Pembroke
Bow |
NH
NH
NH
NH
NH
NH
NH
NH |
US
US
US
US
US
US
US
US |
|
|
Assignee: |
Allegro Microsystems, LLC
(Worcester, MA)
|
Family
ID: |
49510575 |
Appl.
No.: |
13/685,287 |
Filed: |
November 26, 2012 |
Current U.S.
Class: |
324/207.25;
324/202; 324/174 |
Current CPC
Class: |
G01P
3/489 (20130101); G01D 5/2448 (20130101); G01B
7/14 (20130101); G01P 3/488 (20130101) |
Current International
Class: |
G01B
7/30 (20060101) |
Field of
Search: |
;324/207.2,207.21,207.25,174,202 |
References Cited
[Referenced By]
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|
Primary Examiner: Patidar; Jay
Attorney, Agent or Firm: Daly, Crowley, Mofford &
Durkee, LLP
Claims
What is claimed is:
1. A circuit for detecting a movement of an object, comprising: a
magnetic field sensing element for generating a magnetic field
signal proportional to a magnetic field associated with the object,
wherein the magnetic field signal has cycles including a present
cycle; a motion detector configured to generate a motion signal
indicative of the movement of the object, wherein the motion signal
has edges associated with the cycles of the magnetic field signal,
wherein the motion detector comprises: a peak identifying circuit
for tracking at least one of a positive peak of the magnetic field
signal or a negative peak of the magnetic field signal to provide a
peak signal; a peak sample selection module coupled to receive the
peak signal, configured to save samples of the peak signal, and
configured to select a sample of the peak signal associated with a
respective prior cycle of the magnetic field signal to generate a
selected peak signal; a threshold generator coupled to receive the
selected peak signal and configured to generate a threshold signal
as a function of the selected peak signal; and a comparator coupled
to receive the threshold signal, coupled to receive the magnetic
field signal, and configured to compare the threshold signal with
the magnetic field signal in order to generate the motion
signal.
2. The circuit of claim 1, wherein the peak sample selection module
is further configured to select a plurality of samples of the peak
signal associated with a respective plurality of prior cycles of
the magnetic field signal and further comprises a function
processor configured to combine the selected plurality of samples
of the peak signal to generate the selected peak signal.
3. The circuit of claim 2, wherein the function processor is
configured to average the selected plurality of samples of the peak
signal to generate the selected peak signal.
4. The circuit of claim 1, wherein the peak sample selection module
comprises: an analog-to-digital converter coupled to receive the
peak signal and configured to convert the peak signal to digital
samples of the peak signal; a memory coupled to receive the digital
samples and configured to save a plurality of the digital samples;
and a digital-to-analog converter coupled to receive samples
related to selected ones of the plurality of the digital samples to
provide the selected peak signal.
5. The circuit of claim 4, wherein the memory comprises a multi-bit
digital shift register.
6. The circuit of claim 1, wherein the peak sample selection module
comprises: an analog memory coupled to receive the peak signal and
configured to save a plurality of analog samples of the peak
signal; and a circuit module configured to select analog samples
from among the plurality of analog samples to provide the selected
peak signal.
7. The circuit of claim 6, wherein the analog memory comprises an
analog shift register.
8. The circuit of claim 1, wherein the peak sample selection module
comprises a multiplexer configured to select a sample of the peak
signal associated with a present cycle of the magnetic field signal
or to select samples of the peak signal associated with respective
prior cycles of the magnetic field signal in response to a control
signal.
9. The circuit of claim 8 wherein the control signal causes the
multiplexer to select the sample of the peak signal associated with
the present cycle of the magnetic field signal during a calibration
mode of operation before a predetermined number of cycles of the
magnetic field signal have occurred and causes the multiplexer to
select the samples of the peak signal associated with the
respective prior cycles of the magnetic field signal during a
running mode of operation after the predetermined number of cycles
of the magnetic field signal have occurred.
10. The circuit of claim 1, wherein the threshold generator is
configured to generate first and second different threshold signals
based on the state of the motion signal.
11. The circuit of claim 1, wherein the peak identifying circuit
comprises at least one of: a PDAC configured to generate a PDAC
signal as the peak signal to track the magnetic field signal during
a PDAC update time interval and to hold the magnetic field signal
at times outside of the PDAC update time interval; or an NDAC
configured to generate an NDAC signal as the peak signal to track
the magnetic field signal during an NDAC update time interval and
to hold the magnetic field signal at times outside of the NDAC
update time interval.
12. The circuit of claim 1, wherein the threshold generator
comprises a resistor divider coupled to receive the selected peak
signal, and configured to generate the threshold signal.
13. The circuit of claim 12, wherein the threshold generator
comprises: a first voltage source coupled to the resistor divider
and configured to generate a first threshold signal; and a second
voltage source coupled to the resistor divider and configured to
generate a second threshold signal.
14. The circuit of claim 1, wherein the threshold generator
comprises a digital logic circuit.
15. The circuit of claim 1, wherein the magnetic field sensing
element comprises at least two magnetic field sensing elements for
generating a first magnetic field signal and a second magnetic
field signal, wherein the first magnetic field signal has cycles
including a present cycle and the second magnetic field signal has
cycles including a present cycle, wherein the motion detector
comprises first and second motion detectors coupled to receive the
first and second magnetic field signals, respectively, wherein the
first motion detector is configured to generate a first motion
signal indicative of the movement of the object, and wherein the
second motion detector is configured to generate a second motion
signal indicative of the movement of the object.
16. The circuit of claim 1, further comprising a magnet in
proximity to the magnetic field sensing element for generating the
magnetic field.
17. The circuit of claim 16 wherein the magnet, the magnetic field
sensing element, and the motion detector are provided in a single
integrated circuit package.
18. A method of detecting a movement of an object, comprising the
steps of: generating a magnetic field signal proportional to a
magnetic field associated with the object, wherein the magnetic
field signal has cycles including a present cycle; and generating a
motion signal indicative of the movement of the object, wherein the
motion signal has edges associated with the cycles of the magnetic
field signal, wherein the generating the motion signal comprises:
generating a peak signal in accordance with peaks of the magnetic
field signal; saving samples of the peak signal; selecting a sample
of the peak signal associated with a respective prior cycle of the
magnetic field signal; generating a selected peak signal related to
the selected samples of the peak signal; generating a threshold
signal based on the selected peak signal; and comparing the
threshold signal with the magnetic field signal to generate the
motion signal.
19. The method of claim 18, wherein the method further comprises
the steps of: selecting a plurality of samples of the peak signal
associated with a respective plurality of prior cycles of the
magnetic field signal; and combining the plurality of samples of
the peak signal to generate the selected peak signal.
20. The method of claim 19, wherein the combining comprises
averaging the plurality of samples of the peak signal to generate
the selected peak signal.
21. The method of claim 20, wherein the saving comprises converting
the peak signal to digital samples and saving a plurality of the
digital samples, and wherein the generating the selected peak
signal comprises generating the selected peak signal in accordance
with samples related to the selected ones of the plurality of the
digital samples.
22. The method of claim 20, wherein the saving the plurality of the
digital samples comprises saving the plurality of the digital
samples in a multi-bit digital shift register.
23. The method of claim 20, wherein the saving comprises saving
analog samples of the peak signal.
24. The method of claim 23, wherein the saving the analog samples
comprises saving the analog samples of the peak signal in an analog
memory comprising an analog shift register.
25. A method of detecting movement of an object, comprising the
steps of: generating a magnetic field signal with a magnetic field
sensing element, wherein the magnetic field signal is proportional
to a magnetic field associated with the object, wherein the
magnetic field signal has cycles including a present cycle;
generating a peak signal that tracks at least a portion of the
magnetic field signal; using the peak signal to generate a selected
peak signal in accordance with a prior cycle of the magnetic field
signal prior to the present cycle; generating a threshold signal
based on the selected peak signal; and comparing the threshold
signal to the magnetic field signal.
26. The method of claims 25, wherein the using comprises: selecting
a first sample of the peak signal a first predetermined number of
cycles prior to the present cycle; selecting a second sample of the
peak signal a second predetermined number of cycles prior to the
present cycle; and averaging the first and second samples to
generate the selected peak signal.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not Applicable.
FIELD OF THE INVENTION
This invention relates generally to integrated circuits and, more
particularly, to integrated circuits for detecting a movement or a
rotation of a ferromagnetic object.
BACKGROUND OF THE INVENTION
Magnetic field sensors (e.g., rotation detectors) for detecting
ferromagnetic articles and/or magnetic articles are known. The
magnetic field associated with the ferromagnetic article or
magnetic article is detected by a magnetic field sensing element,
such as a Hall element or a magnetoresistance element, which
provides a signal (i.e., a magnetic field signal) proportional to a
detected magnetic field. In some arrangements, the magnetic field
signal is an electrical signal.
The magnetic field sensor processes the magnetic field signal to
generate an output signal that changes state each time the magnetic
field signal crosses a threshold, either near to peaks (positive
and/or negative peaks) or near to some other level, for example,
zero crossings of the magnetic field signal. Therefore, the output
signal has an edge rate or period indicative of a speed of rotation
of the ferromagnetic or magnetic object, for example, a gear or a
ring magnet.
One application for a magnetic field sensor is to detect the
approach and retreat of each tooth of a rotating ferromagnetic
gear, either a hard magnetic gear or a soft ferromagnetic gear that
is back-biased with a magnet. In some particular arrangements, a
ring magnet having magnetic regions (permanent or hard magnetic
material) with alternating polarity is coupled to the ferromagnetic
gear or other rotating device such as a wheel axle or is used by
itself and the magnetic field sensor is responsive to approach and
retreat of the magnetic regions of the ring magnet. In other
arrangements, a gear is disposed proximate to a stationary magnet
and the magnetic field sensor is responsive to perturbations of a
magnetic field as the gear rotates.
In one type of magnetic field sensor, sometimes referred to as a
peak-to-peak percentage detector, one or more threshold levels are
equal to respective percentages of the peak-to-peak magnetic field
signal. One such peak-to-peak percentage detector is described in
U.S. Pat. No. 5,917,320 entitled "Detection of Passing Magnetic
Articles While Periodically Adapting Detection Threshold" and
assigned to the assignee of the present invention.
Another type of magnetic field sensor, sometimes referred to as a
slope-activated detector (or peak-referenced detector), is
described in U.S. Pat. No. 6,091,239 entitled "Detection Of Passing
Magnetic Articles With a Peak Referenced Threshold Detector," also
assigned to the assignee of the present invention. In the
peak-referenced magnetic field sensor, the threshold signal differs
from the positive and negative peaks (i.e., the peaks and valleys)
of the magnetic field signal by a predetermined amount. Thus, in
this type of magnetic field sensor, the output signal changes state
when the magnetic field signal comes away from a peak or valley of
the magnetic field signal by the predetermined amount.
It should be understood that, because the above-described
peak-to-peak percentage threshold detector and the above-described
peak-referenced detector both have circuitry that can identify the
positive and negative peaks of a magnetic field signal, both such
detectors include a circuit portion referred to herein as a "peak
identifier", which is configured to detect positive peaks and/or
negative peaks of the magnetic field signal. The peak-to-peak
percentage threshold detector and the peak-referenced detector,
however, each use the detected peaks in different ways to provide a
so-called "threshold generator," which is configured to use the
identified peaks to generate one or more threshold levels against
which the magnetic field signal can be compared. This comparison
can result in a so-called "PosComp" motion signal that has an edge
rate representative of a speed of movement, e.g., rotation, of the
moving object.
In order to accurately detect the positive and negative peaks of a
magnetic field signal, in some embodiments, the rotation detector
can be capable of tracking at least part of the magnetic field
signal. To this end, typically, one or more digital-to-analog
converters (DACs) can be used to generate a tracking signal, which
tracks the magnetic field signal. For example, in the
above-referenced U.S. Pat. Nos. 5,917,320 and 6,091,239, two DACs
are used, one (PDAC) to detect the positive peaks of the magnetic
field signal and the other (NDAC) to detect the negative peaks of
the magnetic field signal.
Some types of rotation detectors perform one or more types of
initialization or calibration, for example, at a time near to start
up or power up of the rotation detector, or otherwise, from time to
time as desired. During one type of calibration, the
above-described threshold level is determined.
Once the above-described threshold level is initially determined,
various schemes may be used for updating the threshold level to
ensure that the threshold level remains at the desired relationship
with respect to the peak-to-peak magnetic field signal level. For
example, as described in U.S. Pat. No. 6,525,531 entitled
"Detection of Passing Magnetic Articles while Adapting the
Detection Threshold" and assigned to the assignee of the subject
invention, the positive and negative detected peak signals (PDAC
and NDAC, respectively) freely track "outwardly" to follow the
magnetic field signal as it increases above PDAC and decreases
below NDAC, respectively, following which such detected peak
signals are selectively allowed to move "inward" (i.e., PDAC
decreases and NDAC increases) to the level of the magnetic field
signal upon transitions of the PosComp signal. Such threshold
signal updating may be performed following an initial calibration
mode, such as during a "running mode" of operation.
Some moving objects, for example, rotating moving objects, which
are sensed by the above-described magnetic field sensors, exhibit
irregular motions or have irregular features. For example, a gear
may have wobble as it rotates, it may have run out (asymmetry about
its axis of rotation), or it may have irregularities in its
mechanical dimensions, for example, some gear teeth may be wider or
taller than others. Additionally, anomalies in the conditions
associated with the sensor or detected moving objects can cause
intermittent oscillations of the object or other changes in the
magnetic field detection. For example, when the magnetic field
sensor is used to detect wheel speed in an automobile Anti-Lock
Brake System (ABS), potholes can result in temporary changes to the
axis of rotation of a wheel and thus, in the air gap (i.e., the
distance from the object to the magnetic field sensing element).
Such irregularities can cause variations in the magnetic field
signal that can lead to generation of thresholds that are not
ideal, thereby resulting in a PosComp signal that has edges that
are not accurately placed relative to cycles of the magnetic field
signal associated with features of the moving object.
It would, therefore, be desirable to provide a magnetic field
sensor that can accurately identify a threshold level associated
with a magnetic field signal, accurate even in the presence of
irregularities in the motion of, or in the mechanical
characteristics of, the moving object being sensed and/or in the
presence of intermittent conditions associated with the sensor
system.
SUMMARY OF THE INVENTION
According to an aspect of the invention, a circuit for detecting a
movement of an object includes a magnetic field sensing element for
generating a magnetic field signal proportional to a magnetic field
associated with the object and having cycles including a present
cycle and a motion detector configured to generate a motion signal
indicative of the movement of the object and having edges
associated with the cycles of the magnetic field signal. The motion
detector includes a (a) peak identifying circuit for tracking at
least one of a positive peak of the magnetic field signal or a
negative peak of the magnetic field signal to provide a peak
signal, a peak sample selection module coupled to receive the peak
signal, configured to save samples of the peak signal, and
configured to select a sample of the peak signal associated with a
respective prior cycle of the magnetic field signal to generate a
selected peak signal; (b) a threshold generator coupled to receive
the selected peak signal and configured to generate a threshold
signal as a function of the selected peak signal; and (c) a
comparator coupled to receive the threshold signal, coupled to
receive the magnetic field signal, and configured to compare the
threshold signal with the magnetic field signal in order to
generate the motion signal.
The peak sample selection module is configured to select a
plurality of samples of the peak signal associated with a
respective plurality of prior cycles of the magnetic field signal
and further includes a function processor configured to combine the
selected plurality of samples of the peak signal to generate the
selected peak signal. The function processor may be configured to
average the selected plurality of samples of the peak signal to
generate the selected peak signal.
In some embodiments, the peak sample selection module may include
an analog-to-digital converter coupled to receive the peak signal
and configured to convert the peak signal to digital samples of the
peak signal, a memory coupled to receive the digital samples and
configured to save a plurality of the digital samples, and a
digital-to-analog converter coupled to receive samples related to
selected ones of the plurality of the digital samples to provide
the selected peak signal. In other embodiments, the peak sample
selection module may include an analog memory coupled to receive
the peak signal and configured to save a plurality of analog
samples of the peak signal and a circuit module configured to
select analog samples from among the plurality of analog samples to
provide the selected peak signal.
The peak sample selection module may include a multiplexer
configured to select a sample of the peak signal associated with a
present cycle of the magnetic field signal or to select samples of
the peak signal associated with respective prior cycles of the
magnetic field signal in response to a control signal. The control
signal may cause the multiplexer to select the sample of the peak
signal associated with the present cycle of the magnetic field
signal during a calibration mode of operation before a
predetermined number of cycles of the magnetic field signal have
occurred and causes the multiplexer to select the samples of the
peak signal associated with the respective prior cycles of the
magnetic field signal during a running mode of operation after the
predetermined number of cycles of the magnetic field signal have
occurred.
Features include the threshold generator being configured to
generate first and second different threshold signals based on the
state of the motion signal and the peak identifying circuit
including at least one of a PDAC or an NDAC. The threshold
generator may include a resistor ladder coupled to receive the PDAC
signal at a first end, coupled to receive the NDAC signal at a
second end, and configured to generate the threshold signal at an
intermediate tap between the first and second ends. In other
embodiments, the threshold generator may include a first voltage
source coupled to receive the PDAC signal and configured to
generate a first threshold signal and a second voltage source
coupled to receive the NDAC signal and configured to generate a
second threshold signal.
In some embodiments, the magnetic field sensing element comprises
at least two magnetic field sensing elements. A magnet may be
provided in proximity to the magnetic field sensing element for
generating the magnetic field. In some embodiments, the magnet, the
magnetic field sensing element, and the motion detector are
provided in a single integrated circuit package.
According to a further aspect of the invention, a method of
detecting a movement of an object includes generating a magnetic
field signal proportional to a magnetic field associated with the
object, wherein the magnetic field signal has cycles including a
present cycle and generating a motion signal indicative of the
movement of the object and having edges associated with the cycles
of the magnetic field signal. Generating the motion signal includes
generating a peak signal in accordance with peaks of the magnetic
field signal, saving samples of the peak signal, selecting a sample
of the peak signal associated with a respective prior cycle of the
magnetic field signal, generating a selected peak signal related to
the selected samples of the peak signal, generating a threshold
signal based on the selected peak signal, and comparing to the
threshold signal with the magnetic field signal to generate the
motion signal.
Additional steps may include selecting a plurality of samples of
the peak signal associated with a respective plurality of prior
cycles of the magnetic field signal and combining the plurality of
samples of the peak signal to generate the selected peak signal. In
some embodiments, combining includes averaging the plurality of
samples of the peak signal to generate the selected peak
signal.
Another method of detecting movement of an object includes
generating a magnetic field signal with a magnetic field sensing
element, wherein the magnetic field signal is proportional to a
magnetic field associated with the object and has cycles including
a present cycle, generating a peak signal that tracks at least a
portion of the magnetic field signal, using the peak signal to
generate a selected peak signal in accordance with a prior cycle of
the magnetic field signal prior to the present cycle, generating a
threshold signal based on the selected peak signal, and comparing
the threshold signal to the magnetic field signal. Generating the
selected peak signal may include selecting a first sample of the
peak signal a first predetermined number of cycles prior to the
present cycle, selecting a second sample of the peak signal a
second predetermined number of cycles prior to the present cycle,
and averaging the first and second samples to generate the selected
peak signal.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing features of the invention, as well as the invention
itself may be more fully understood from the following detailed
description of the drawings, in which:
FIG. 1 is block diagram showing an exemplary magnetic field sensor
in the form of a rotation sensor, having a motion detector with a
threshold generator and comparator circuit;
FIG. 1A is a block diagram showing another exemplary magnetic field
sensor in the form of a rotation sensor, having two motion
detectors each with a respective threshold generator and comparator
circuit;
FIG. 2 is a block diagram showing an exemplary motion detector that
can be used as the motion detector of FIG. 1, having two
digital-to-analog converters (DACs), a positive DAC (PDAC) and a
negative DAC (NDAC);
FIG. 2A is a block diagram showing two exemplary motion detectors
that can be used as the two motion detectors of FIG. 1A, having two
respective PDACs and two respective NDACs;
FIG. 2B is a block diagram of another exemplary magnetic field
sensor in the form of a rotation sensor and having a zero crossing
detector;
FIG. 3 is a graph showing a magnetic field signal, with associated
PDAC and NDAC signals and thresholds, and the resulting PosComp
signal;
FIG. 3A is a graph showing two magnetic field signals, each at a
different revolution of a moving object being sensed, with
associated PDAC and NDAC signals and thresholds;
FIG. 4 is a graph showing two magnetic field signals, each at a
different revolution of a moving object being sensed, and each
having a different DC offset voltage;
FIG. 5 is a block diagram of an exemplary motion detector that can
be used as the motion detectors of FIGS. 1, 1A, 2, and 2A, having
an analog peak identifier, a peak sample selection module, a
threshold generator, and an analog comparator;
FIG. 6 is a block diagram of another exemplary motion detector that
can be as the motion detectors of FIGS. 1, 1A, 2, and 2A, having a
digital peak identifier, a peak sample selection module, a
threshold generator, and a digital comparator;
FIG. 7 is a block diagram of an exemplary threshold generator that
can be used as the threshold generator of FIGS. 5 and 6;
FIG. 8 is a block diagram of yet another exemplary threshold
generator that can be used as the threshold generator of FIGS. 5
and 6; and
FIG. 9 is a flow diagram showing an illustrative method for
generating thresholds for a magnetic field sensor.
DETAILED DESCRIPTION OF THE INVENTION
Before describing the present invention, some introductory concepts
and terminology are explained. As used herein, the term "magnetic
field sensing element" is used to describe a variety of types of
electronic elements that can sense a magnetic field. The magnetic
field sensing elements can be, but are not limited to, Hall effect
elements, magnetoresistance elements, or magnetotransistors. As is
known, there are different types of Hall effect elements, for
example, planar Hall elements, vertical Hall elements, and Circular
Vertical Hall (CVH) elements. As is also known, there are different
types of magnetoresistance elements, for example, semiconductor
magnetoresistance elements such as and Indium antimonide (InSb)
elements, anisotropic magnetoresistance (AMR) elements, giant
magnetoresistance (GMR) elements, tunneling magnetoresistance (TMR)
elements, and magnetic tunnel junction (MTJ) elements.
As is known, some of the above-described magnetic field sensing
elements tend to have an axis of maximum sensitivity parallel to a
substrate that supports the magnetic field sensing element, and
others of the above-described magnetic field sensing elements tend
to have an axis of maximum sensitivity perpendicular to a substrate
that supports the magnetic field sensing element. In particular,
most, but not all, types of magnetoresistance elements tend to have
axes of maximum sensitivity parallel to the substrate and most, but
not all, types of Hall elements tend to have axes of sensitivity
perpendicular to a substrate.
As used herein, the term "magnetic field sensor" is used to
describe a circuit that includes a magnetic field sensing element.
Magnetic field sensors are used in a variety of applications,
including, but not limited to, a current sensor that senses a
magnetic field generated by a current carried by a current-carrying
conductor, a magnetic switch or proximity detector that senses the
proximity of a ferromagnetic or magnetic object, a rotation
detector that senses passing ferromagnetic articles, for example,
magnetic domains of a ring magnet or teeth or slots of a
ferromagnetic gear, and a magnetic field sensor that senses a
magnetic field density of a magnetic field. Rotation detectors are
used as examples herein. However, the circuits and techniques
described herein apply also to any magnetic field sensor capable of
detecting a motion of an object.
Peak-to-peak threshold detectors and peak-referenced detectors are
described above. As used herein, the term "tracking circuit" or
alternatively "peak identifier" is used to describe a circuit that
can track and perhaps hold a signal representative of a positive
peak or a negative peak (or both) of a magnetic field signal. It
should be understood that both a peak-to-peak percentage threshold
detector and a peak-referenced detector can both employ a tracking
circuit or peak identifier. As used herein, the term "threshold
generator" is used to describe any circuit configured to generate a
threshold. As used herein, the term "comparator" is used to
describe any circuit capable of comparing two or more signals,
which can be analog or digital signals. Thus, a comparator can be,
but is not limited to, an analog comparator configured to compare
analog signals, a digital comparator configured to compare digital
signals, or a programmable device, for example, a microprocessor
having code therein for comparing two digital signals.
While circuits are shown below that use peak-to-peak percentage
threshold detectors, in other embodiments, similar circuits can use
peak-referenced detectors. Also, while circuits are shown below
that use rotation detectors, in some embodiments, the rotations
detectors can be motion detectors configured to detect other
motions of an object, for example, linear motions.
Operation of a magnetic field sensor in a so-called "calibration
mode," also referred to herein as an "initialization mode," is
described herein. Reference is also made herein to operation of a
magnetic field sensor in a so-called "running mode." The
calibration mode can occur at the beginning of operation (or from
time to time as desired) and the running mode is achieved at other
times.
In general, during the calibration mode, an output signal from the
magnetic field sensor may not be accurate, and during the running
mode, the output signal is considered to be accurate, i.e., it has
edges indicative of features of the magnetic field signal.
While a calibration time period is discussed herein, an end of
which ends the calibration mode discussed herein in accordance with
certain criteria, it should be recognized that other calibrations
can be performed after the end of the indicated calibration time
period. For example, an automatic gain control can continue
calibrating after the end of the indicated calibration time period.
At some point after the end of the indicated calibration time
period, but not necessarily coincident with the end of the
indicated calibration time period, the magnetic field sensors
described herein can enter the running mode, during which updates
to values of circuit parameters can be achieved in a different way
than during the calibration mode. Such updates during the running
mode can include updates to the threshold level signal according to
circuits and methods described herein.
Referring now to FIG. 1, an exemplary magnetic field sensor 10
includes a magnetic field sensing element 14 for generating a
signal 14a, 14b (i.e., a magnetic field signal) proportional to a
magnetic field associated with an object or target 24. The magnetic
field sensing element 14 can be, but is not limited to, a Hall
effect element, a magnetoresistance element, or a
magnetotransistor.
It should be understood that the object 24 need not be a part of
the magnetic field sensor 10. The object 24 can be an object
configured to rotate, for example, a ferromagnetic gear or ring
magnet. The magnetic field sensor 10 can include a permanent magnet
16 disposed proximate to the magnetic field sensing element 14. In
some embodiments, the magnetic field sensor 10 can be implemented
as a packaged integrated circuit, and may contain the motion
detector 12 and the magnet 16.
The magnetic field sensor 10 can include an amplifier 18 coupled to
receive the signal 14a, 14b from the magnetic field sensing element
14 and configured to generate a signal 18a (also a magnetic field
signal).
The magnetic field sensor 10 can also include a motion detector,
here a rotation detector 12, having an amplifier 20 coupled to
receive the signal 18a and configured to generate a signal 20a,
also referred to herein as a DIFF signal, representative of the
signal 18a. In some embodiments, the amplifier 20 is an automatic
gain control (AGC) amplifier. The DIFF signal 20a is also referred
to herein as a magnetic field signal. Thus, the signals 14a, 14b,
18a, and 20a are all magnetic field signals, and are all indicative
of and proportional to a magnetic field experienced by the magnetic
field sensing element 14 and associated with the object.
The rotation detector 12 can include a threshold generator and
comparator circuit 22 coupled to receive the DIFF signal 20a and
configured to generate a PosComp "motion signal" 22a indicative of
a movement (i.e., rotation) of the object 24. In some embodiments,
the motion signal 22a is a two state square wave having a frequency
proportional to a speed of rotation of the object 24.
The magnetic field sensing element 14 can be responsive to motion
of the object 24, for example, motion of ferromagnetic gear teeth
or other features of associated with a gear, of which gear teeth
24a-24c upon a gear 24 are representative. To this end, the fixed
magnet 16 can be disposed proximate to the magnetic field sensing
element 14 and the gear teeth can disturb the magnetic field
generated by the magnet 16 as the gear rotates. In other
arrangements, the magnetic field sensing element 14 can be
responsive to motion of magnetic regions 24a-24c upon a magnet 24,
for example, magnetic regions of a ring magnet that is coupled to a
gear or some other rotating structure such as a wheel axle. In some
particular arrangements, the ring magnet 24 and a gear or axle are
coupled together with a shaft or the like. In these particular
arrangements, the ring magnet 24 can be proximate to the magnetic
field sensing element 14, but the gear or wheel axle need not be
proximate to the magnetic field sensing element 14. The features
24a-24c of the target object 24 will be referred to generally
herein as "target regions" and the object 24 may be referred to as
a target.
The magnetic field sensing element 14 is responsive to proximity of
the target regions 24a-24c. In operation, the magnetic field
sensing element 14 produces the magnetic field signal 14a, 14b (and
also the magnetic field signals 18a, 20a) that may have a generally
sinusoidal shape when the target 24 rotates, wherein each peak
(positive and negative) of the sinusoid is associated with one of
the target regions 24a-24c. Alternatively, the magnetic field
signal 14a, 14b may have a substantially square-wave shape.
The magnetic field sensor 10 can also include an output protocol
processor 26 coupled to receive the PosComp motion signal 22a and
configured to generate an output signal 26a representative of the
speed of rotation of the object 24. In some embodiments, the output
signal 26a is a two state square wave having a frequency
proportional to the speed of rotation of the object 24. In other
embodiments, the output signal 26a comprises digital words
representative of the speed of rotation of the object 24.
Referring now to FIG. 1A, in which like elements of FIG. 1 are
shown having like reference designations, another exemplary
magnetic field sensor 50 includes a plurality of magnetic field
sensing elements 52a-52c for generating signals 52aa, 52ab, 52ba,
52bb, 52ca, 52cb (magnetic field signals) proportional to a
magnetic field.
The magnetic field sensor 50 includes a right channel amplifier 58
coupled to the magnetic field sensing elements 52a and 52b and
configured to generate a signal 58a (also a magnetic field signal).
The magnetic field sensor 50 also includes a left channel amplifier
64 coupled to the magnetic field sensing elements 52b and 52c and
configured to generate a signal 64a (also a magnetic field signal).
The signal 58a is proportional to a magnetic field at a first
location relative to the object 24 and the signal 64a is
proportional to a magnetic field at a second location relative to
the object 24. The first and second locations are associated with
right and left electronic channels, respectively.
The magnetic field sensor 50 also includes motions detectors, here
rotation detectors 56, which includes right and left channel motion
detectors, here rotation detectors 56a, 56b, respectively. The
rotation detector 56a can include an amplifier 60 coupled to
receive the signal 58a and configured to generate an RDIFF signal
60a (also a magnetic field signal) representative of the signal
58a. The rotation detector 56b can include an amplifier 66 coupled
to receive the signal 64a and configured to generate an LDIFF
signal 66a (also a magnetic field signal) representative of the
signal 64a. In some embodiments, the amplifiers 60, 66 are
automatic gain control (AGC) amplifiers.
The rotation detector 56a also includes a right channel threshold
generator and comparator circuit 62 coupled to receive the RDIFF
signal 60a and configured to generate an RPosComp motion signal 62a
indicative of a movement (i.e., rotation) of the object 24. The
rotation detector 56b also includes a left channel threshold
generator and comparator circuit 68 coupled to receive the LDIFF
signal 66a and configured to generate an LPosComp motion signal 68a
indicative of the movement (i.e., rotation) of the object 24.
In some embodiments, the motion signals 62a, 68a are each two state
square waves having a frequency proportional to the speed of
rotation of the object 24. It will be understood that, since the
magnetic field sensing elements 52a-52c are at different physical
locations, the RPosComp signal 62a can have a different phase than
the LPosComp signal 68a. Furthermore, if the object 24 rotates in
one direction, the phase of the RPosComp 62a will lead the phase of
the LPosComp signal 68a, but if the object 24 rotates in the
opposite direction, the phase relationship will reverse. Therefore,
the magnetic field sensor 50, unlike the magnetic field sensor 10
of FIG. 1, is able to generate signals representative not only of
the speed of rotation of the object 24, but also signals
representative of the direction of rotation of the object 24.
The above designations "left" and "right" (also L and R,
respectively) are indicative of physical placement of the magnetic
field sensors 52a-52c relative to the object 24 and correspond
arbitrarily to left and right channels. In the illustrative
embodiment, three magnetic field sensing elements 52a-52c are used
for differential magnetic field sensing, with the central sensor
52b used in both channels. While three magnetic field sensors
52a-52c are shown, it should be appreciated that two or more
magnetic field sensors can be used. For example, in an embodiment
using only two magnetic field sensors 52a, 52c, only magnetic field
sensor 52a can be coupled to the right channel amplifier 58 and
only the magnetic field sensor 54c can be coupled to the left
channel amplifier 64.
The magnetic field sensor 50 can also include an output protocol
processor 70 coupled to receive the RPosComp signal 62a and the
LPosComp signal 68a and configured to generate an output signal 70a
representative of at least the speed of rotation of the object 24.
In some embodiments, the output signal 70a is also representative
of the direction of rotation of the object 24.
In some embodiments the output signal 70a is a two state square
wave having a frequency proportional to the speed of rotation of
the object 24 and a duty cycle (or pulse width or on-time duration)
representative of the direction of the rotation of the object 24.
In other embodiments, the output signal 70a comprises digital words
representative of the speed of rotation of the object 24 and the
direction of rotation.
Referring now to FIG. 2, in which like elements of FIG. 1 are shown
having like reference designations, a circuit 100 includes an
exemplary rotation (motion) detector 102, which can be the same as
or similar to the rotation detector 12 of FIG. 1, but shown in
greater detail.
The rotation detector 102 is coupled to receive the magnetic field
signal 18a of FIG. 1. The magnetic field signal 18a can include an
undesirable DC offset. Therefore, an auto offset controller 104, an
offset digital-to-analog converter (DAC) 106, and a summer 108 can
be provided in order to reduce or eliminate the DC offset.
The rotation detector 102 can also include an automatic gain
control (AGC) amplifier 112 coupled to receive an output signal
108a generated by the summer 108 and configured to generate the
DIFF signal 20a having an amplitude within a controlled amplitude
range. It should be understood that the DIFF signal 20a is
representative of the magnetic field experienced by one or more
magnetic field sensing elements, for example, the magnetic field
sensing element 14 of FIG. 1.
The DIFF signal 20a is coupled to a comparator 114 (a comparator
part 116b of the threshold generator and comparator circuit 116).
The comparator 114 also receives a threshold signal 138. Generation
of the threshold signal 138 is further described below. The
threshold comparator 114 is configured to generate the PosComp
signal 22a.
The threshold signal 138 can switch between two different values.
In one particular embodiment, the threshold signal 138 can be
determined by a threshold detector 116a (a threshold generator part
116a of the threshold generator and comparator circuit 116). A
first threshold signal 132a, sometimes referred to as an operate
threshold, can be a first predetermined percentage e.g.,
eighty-five percent, of a peak-to-peak magnitude of the DIFF signal
20a, e.g., near to but below a positive peak of the DIFF signal
20a. A second threshold signal 132b, sometimes referred to as a
release threshold, can be a second predetermined percentage, e.g.,
fifteen percent, of a peak-to-peak magnitude of the DIFF signal
20a, e.g., near to but above a negative peak of the DIFF signal
20a. The threshold signal 138 can, therefore, be relatively near to
and below a positive peak of the DIFF signal 20a at some times and
relatively near to and above a negative peak of the DIFF signal 20a
at other times. Therefore, the comparator 114 can generate the
PosComp signal 22a having edges closely associated with the
positive and negative peaks of the DIFF signal 20a.
However, in other embodiments, the threshold signal 138 can take on
two other different values, for example, two values near to zero
crossings of the DIFF signal 20a, and therefore, the threshold
comparator 114 can generate the PosComp signal 22a having edges
closely associated with the zero crossings of the DIFF signal 20a.
In still other embodiments, the threshold signal 138 can take on
two other different values as may be generated, for example, by a
peak-referenced detector, which is described above.
The threshold signal (or voltage) 138 is generated by the threshold
generator and comparator circuit 116, which can be the same as or
similar to the threshold generator and comparator circuit 22 of
FIG. 1.
The threshold generator part 116a of the threshold generator and
comparator circuit 116 can include counters 120, 122, a PDAC 124,
an NDAC, 126, first and second comparators 128, 130, respectively,
an update logic circuit 118, a resistor ladder 132, and first and
second switches 134, 136, respectively. The PDAC 124 is coupled to
receive a count signal 120a from the counter 120. The PDAC 124 is
configured to generate a PDAC output signal 124a coupled to a first
end of the resistor ladder 132. The NDAC 126 is coupled to receive
a count signal 122a from the counter 122. The NDAC 126 is
configured to generate an NDAC output signal 126a coupled to a
second end of the resistor ladder 132. The PDAC output signal 124a
and the NDAC output signal 126a are also referred to herein as peak
tracking signals or simply tracking signals.
In operation, the PDAC output signal 124a can sometimes track the
DIFF signal 20a and sometimes hold a positive peak of the DIFF
signal 20a and the NDAC output signal 126a can sometimes track the
DIFF signal 20a and sometimes hold a negative peak of the DIFF
signal 20a.
The first switch 134 is coupled to receive a first threshold signal
132a signal from a first tap of the resistor ladder 132 and the
second switch 136 is coupled to receive a second threshold signal
132b signal from a second tap of the resistor ladder 132. The first
switch 134 can be controlled by the PosComp signal 22a and the
second switch 136 can be controlled by an inverted PosComp signal
22a, i.e. a PosCompN signal.
The first comparator 128 is coupled to receive the PDAC signal 124a
and also coupled to receive the DIFF signal 20a and configured to
generate a first feedback signal. The second comparator 130 is
coupled to receive the NDAC signal 126a and also coupled to receive
the DIFF signal 20a and configured to generate a second feedback
signal 130a.
Referring now to FIG. 2A, in which like elements of FIG. 1A are
shown having like reference designations, a circuit 150 includes
two exemplary rotation (motion) detectors 152, identified as 152a,
152b, which can be the same as or similar to the rotation detectors
56a, 56b of FIG. 1A, but shown in greater detail.
The rotation detectors 152 can include two threshold generator and
comparator circuits 116, 164, which can be the same as or similar
to the threshold generator and comparator circuit 62, 68 of FIG.
1A, but shown in greater detail. The rotation detector 152a is
coupled to receive the magnetic field signal 58a of FIG. 1A and the
rotation detector 152b is coupled to receive the magnetic field
signal 64a of FIG. 1A. The rotation detector 152a is configured to
generate the RPosComp signal 62a (FIG. 1A) and the RDIFF signal 60a
(FIG. 1A), and the rotation detector 152b is configured to generate
the LPosComp signal 68a (FIG. 1A) and the LDIFF signal 66a (FIG.
1A).
Operation of each one of the two rotation detectors 152a, 152b is
the same as or similar to operation of the rotation detector 102 of
FIG. 2, so is not discussed here again.
Referring now to FIG. 2B, a so-called "zero-crossing detector" 200,
a threshold generator and comparator circuit, can be compared with
the threshold generator and comparator circuit 116 of FIG. 2. Here,
an amplifier 206 is coupled to receive signals 202a, 202b, 204a,
204b from two magnetic field sensing elements 202, 204. The
amplifier is configured to generate a differential output signal
206a, 206b coupled to a band pass filter (BPF) 208. The
differential signal 206a, 206b is comparable to a differential DIFF
signal. The BPF 208 is configured to generate a differential
filtered signal 208a, 208b. A comparator is coupled to receive the
differential filtered signal 208a, 208b and configured to generate
a motion signal, PosComp 210a.
In operation, the signals 208a, 208b essentially operate as
thresholds. The signals 208a, 208b cross each other at or near a
zero crossing of each respective signal 208a, 208b.
Referring now to FIG. 3, a graph 212 has a horizontal axis with a
scale in arbitrary units of time, which can be related to rotation
angle or linear displacement of the object 24 and a vertical axis
with a scale in arbitrary units of voltage, which can be related to
magnetic field strength (Gauss) or an associated digital value. The
waveform 214 can be a DIFF signal representative, for example, of
the DIFF signal 20a of FIGS. 1 and 2 with cycles of the signal 214
being indicative of target regions 24a-24c of FIG. 1A passing the
magnetic field sensing elements 52a-52c of FIG. 1A. In normal
operation, the PDAC signal 222 can reach and acquire positive peaks
of the DIFF signal 214. Similarly, an NDAC signal 224 can reach and
acquire negative peaks of the DIFF signal 214. Thresholds 216a-216h
can be calculated during cycles of the DIFF signal 214. The
thresholds 216a-216h may correspond, for example, to threshold
signals as may be taken from taps of a threshold generator resistor
ladder.
According to an aspect of the invention, one or more thresholds are
generated on the basis of samples of a peak signal (PDAC and/or
NDAC) taken during one or more prior cycles of the DIFF signal 214.
In one particular embodiment, each of a first predetermined number
of thresholds, such as thresholds 216a-216f, are based on peak
signal samples taken during the respective cycle of the DIFF signal
214; whereas, after the predetermined number of DIFF signal cycles
has occurred, each threshold 216g-216h for example is based on a
mathematical combination such as an average of the peak signal
samples taken during prior cycles (and possibly also the present
cycle) of the DIFF signal 214. For example, thresholds 216a and
216b may be based on peak signal samples taken during the first
shown DIFF signal cycle, threshold levels 216c and 216d may be
based on peak signal samples taken during the second shown DIFF
signal cycle, and so forth until a predetermined signal cycle, such
as a fourth signal cycle. Arrows, of which arrow 218 is
representative, illustrate use of a PDAC signal sample from the
present DIFF signal cycle to establish the threshold for the
present cycle. The threshold levels 216g and 216h used during the
fourth DIFF signal cycle are established by averaging the peak
signal samples taken during each of the previous four signal cycles
(i.e., the present cycle and the three prior cycles). Arrow 219
illustrates use of peak signal samples from present and prior
cycles of the DIFF signal to generate threshold 216g.
While the DIFF signal 214 of FIG. 3 is used to illustrate
embodiments in which the threshold signal is based on an average of
peak signal samples from a predetermined number of prior DIFF
signal cycles, it is also possible to establish the threshold
signal based on DIFF signal cycles from a prior revolution of the
target as will be explained with reference to FIG. 3A. Furthermore,
it is also possible to establish the threshold signal based on a
mathematical combination of prior threshold signals (as opposed to
a mathematical combination of previous peak signal samples) as is
also explained with reference to FIG. 3A.
Also shown in FIG. 3 is the PosComp signal that transitions each
time the DIFF signal 214 crosses a threshold 216a-216h as
shown.
Referring now to FIG. 3A, a graph 220 includes two parts 220a,
220b, each part having a horizontal axis with a scale in arbitrary
units of time, which can be related to rotation angle or linear
displacement of the object 24, and a vertical axis with a scale in
arbitrary units of voltage or current, which can be related to
magnetic field strength (Gauss) or an associated digital value.
Cycles of each one of the parts 220a, 220b are indicative of target
regions, e.g., the gear teeth 24a-24c of FIG. 1A, passing by the
magnetic field sensing elements, e.g., the magnetic field sensing
elements 52a-52c of FIG. 1A. The parts 220a, 220b are each
indicative of a different revolution, and at the same positions
(rotational angles), of the target object 24 of FIG. 1.
The part 220b includes a DIFF signal 230b representative, for
example, of the DIFF signal 20a of FIGS. 1 and 2. The DIFF signal
230b is representative of an nth revolution of the object 24 of
FIG. 1. In normal operation, a PDAC signal 222, which is similar to
the PDAC signal 124a of FIG. 2, can reach and acquire positive
peaks of the DIFF signal 230b. Similarly, an NDAC signal 224, which
is similar to the NDAC signal 126a of FIG. 2, can reach and acquire
negative peaks of the DIFF signal 230b.
The part 220a includes a DIFF signal 230a also representative, for
example, of the DIFF signal 20a of FIGS. 1 and 2. The DIFF signal
230a is representative of an (n-1)st revolution, i.e., a prior
revolution, of the target 24 of FIG. 1. In normal operation, the
PDAC signal 222 can reach and acquire positive peaks of the DIFF
signal 230a. Similarly, an NDAC signal 224 can reach and acquire
negative peaks of the DIFF signal 230a.
Thresholds 226a-226f can be calculated during cycles of the DIFF
signal 230a on the (n-1)st revolution of the target 24. Thresholds
228a-228f can be calculated during cycles of the DIFF signal 230b,
but on the nth revolution of the target 24. The thresholds
226a-226f correspond, for example, to threshold signals as may be
taken from a center tap of the resistor ladder 132 of FIG. 2, i.e.,
a 50% point between the positive and negative peaks of the DIFF
signal 230a on the (n-1)st revolution of the target 24. The
thresholds 228a-228f correspond, for example, to threshold signals
as may be taken from the center tap of the resistor ladder 132 on
the nth revolution of the target 24.
Arrows, of which an arrow 232 is representative, indicate that
during the nth revolution of the target represented by the DIFF
signal 230b, during which the thresholds 228a-228f could otherwise
be used, instead, the thresholds 226a-226f are used. On an nth
revolution of the target 24, the threshold determined during an
(n-1)st revolution is used, shifted by one edge. In other words, on
the nth revolution, threshold 226b is used instead of threshold
228a, threshold 226c is used instead of threshold 228b, and so on.
A threshold from a prior cycle is used, but the threshold is used
that is associated with the next edge of the target 24.
Similarly, on an (n+1)st revolution of the target 24, for which a
DIFF signal is not shown, the thresholds 228a-228f could be used.
Thus, thresholds are used from a prior revolution of the target
24.
The waveforms of FIG. 3A illustrate use of thresholds that are
based on thresholds generated during present and/or prior DIFF
signal cycles; whereas the waveforms of FIG. 3 illustrate use of
thresholds that are based on PDAC or NDAC signal samples taken
during present and/or prior DIFF signal cycles. Thus, it will be
appreciated that thresholds may be based on peak signal samples
taken during present and/or prior target revolutions and/or DIFF
signal cycles.
It will be apparent that, on the nth revolution of the target 24,
while use of only thresholds from the (n-1)st revolution is shown,
in other embodiments, any combination of thresholds from prior and
present cycles and revolutions could be used. For example, in one
embodiment, several prior thresholds associated with the same
target region as the present target region now at the nth
revolution can be averaged. For example, thresholds associated with
the same target region but at the (n-1)th, (n-2)th, . . . , (n-M)th
revolutions can be averaged to provide a threshold to be used for
the same target region at the nth revolution.
In still other embodiments, prior thresholds associated with more
than one target region at the current, nth, revolution can be used.
For example, thresholds associated with the different target region
(n-1)th, (n-2)th, . . . , (n-N)th target regions, all in the nth
revolution can be averaged to provide a threshold to be used for a
target region at the nth revolution.
In still other embodiments, not only prior thresholds, but also the
presently determined threshold can be used in either of the above
two averages. Furthermore, while averages are discussed above, any
combination of the thresholds can be used. The combinations can
include, but are not limited to, RMS combinations and weighted
averages.
In still other embodiments, any combination of present and prior
thresholds from present and prior cycles and/or revolutions can be
used.
All of the above-described concepts of using thresholds from prior
cycles and/or revolutions can be similarly applied to using peak
samples (rather than thresholds) from prior cycles and/or
revolutions to generate thresholds.
Accurate threshold placement and resulting edge timing accuracy of
the PosComp signals 62a, 68a of FIG. 2A is important in
applications where the edges are used to represent exact rotational
angle of an object. Such accuracy may be important when the
rotation (motion) detectors 152 of FIG. 2A, are used, for example,
to sense rotation of a camshaft in an automobile in order to
control various engine timings, to sense drive shaft position for
proper transmission operation and/or to sense rotation of wheels in
an automobile ABS system. More generally however, the described
circuitry and techniques are applicable to any applications that
would benefit from threshold accuracy, including without limitation
automotive and industrial motor or positioning applications such as
servo motor control.
Referring now to FIG. 4, a graph 250 has a horizontal axis with a
scale in arbitrary units of time and a vertical axis with a scale
in arbitrary units of voltage. The graph 250 includes a DIFF signal
252 and a DIFF signal 254, each representative, for example, of the
DIFF signal 20a of FIGS. 1 and 2, but each on a different
revolution of the gear 24 of FIGS. 1 and 2. A DC offset 260 is
shown between the two DIFF signals 252, 254. In accordance with the
DC offset 260, different thresholds, e.g., thresholds 256, 258 are
determined on each cycle, not yet taking into account any threshold
corrections.
By sensing the offset change 260, only available since a history of
the thresholds from a plurality of revolutions is stored using
techniques described below, a change or drift of the offset can be
calculated. The offset change can be applied to the threshold used
at each gear tooth (e.g., to thresholds 226a-226f and 228a-228f of
FIG. 3) in order to even more accurately position the
thresholds.
Referring now to FIG. 5, a circuit 300 (i.e., a motion detector)
for detecting a movement of an object includes at least one
magnetic field sensing element (not shown, e.g., 52a-52c of FIG.
1A) for generating DIFF signal 306a proportional to a magnetic
field associated with the object (e.g., the gear 24 of FIGS. 1-1A),
wherein the DIFF signal 306a has cycles including a present cycle.
The circuit 300 generates a motion signal 308a indicative of the
movement of the object, wherein the motion signal 308a has edges
associated with the cycles of the DIFF signal 306a.
The motion detector 300 can include a peak identifier circuit 322
coupled to receive and to track portions of the DIFF signal 306a,
such as positive and negative peaks in order to provide positive
and negative peak tracking signals, or simply peak signals, PDAC
322a and NDAC 322b, respectively.
The motion detector 300 can also include at least one peak sample
selection module 326 coupled to receive a peak tracking signal,
such as the PDAC signal 322a, configured to save samples 332 of the
PDAC signal 322a, configured to select saved samples 332 of the
PDAC signal associated with at least one prior cycle of the DIFF
signal 306a, and configured to generate a selected peak signal 326a
related to the selected samples 332 of the PDAC signal 322a. Since
the peak sample selection module 326 is responsive to the positive
peak tracking signal 322a, such module 326 may be referred to as
the positive peak sample selection module that generates a positive
selected peak signal 326a.
In some embodiments, the motion detector 300 can include a second,
negative peak sample selection module 344 coupled to receive a
different peak tracking signal, such as the NDAC signal 322b,
configured to save samples 352 of the NDAC signal 322b, configured
to select saved samples 352 of the NDAC signal associated with at
least one prior cycle of the DIFF signal 306a, and configured to
generate a selected peak signal 344a related to the selected
samples 352 of the NDAC signal 322b. Since the peak sample
selection module 344 is responsive to the negative peak tracking
signal 322b, such module 344 may be referred to as the negative
peak sample selection module that generates a negative selected
peak signal 344a. Details and operation of the peak sample
selection module 344 are similar to the peak sample selection
module 326.
The motion detector 300 can also include a threshold generator
circuit 380 that is responsive to the positive selected peak signal
326a, the negative selected peak signal 344a (in those embodiments
containing the negative peak sample selection module 344), and that
provides a threshold signal 316. The threshold generator circuit
380 computes the threshold signal 316 based on the selected peak
signals 326a and 344a, which threshold signal 316 may provide
threshold signals 216a-216h of FIG. 3. The threshold generator 380
may include a resistor divider 382 having a first tap 382a coupled
to a first switch 384 and a second tap 382b coupled to a second
switch 386. The first switch 384 is controlled by the PosComp
signal 308a and the second switch 386 is controlled by an inverted
version, N-PosComp, of the PosComp signal. With this arrangement,
the threshold signal 316 is provided by the tap 382a at a first
level corresponding to a first percentage of the difference between
the positive selected peak signal 326a and the negative selected
peak signal 344a when the DIFF signal 306a exceeds the threshold
signal 316 and by the tap 382b at a second level corresponding to a
second percentage of the difference between the positive selected
peak signal 326a and the negative selected peak signal 344a when
the DIFF signal is less than the threshold signal 316.
In embodiments in which the negative peak sample selection module
344 is not present, the NDAC signal 322b can be coupled directly to
the resistor divider 382 in place of the negative selected peak
signal 344a. It will be appreciated by those of ordinary skill in
the art that arrangements are possible for generating the threshold
signal 316 from just one or the other of the PDAC and NDAC signals
or of the positive selected peak signal and negative selected peak
signal, respectively. As one example, the positive selected peak
signal 326a can be coupled to the resistor divider and the other
end of the resistor divider can be coupled to a reference
potential. A tap of the resistor divider can provide a threshold
signal as a percentage of the coupled signal (e.g., the positive
selected peak signal) and the other, non-coupled signal (e.g., the
negative selected peak signal) can be estimated by taking the
inverse of the measured signal.
The motion detector 300 includes a comparator 308 coupled to
receive the threshold signal 316 and the DIFF signal 306a,
configured to compare the threshold signal 316 with the DIFF signal
306a, and configured to generate the motion signal 308a.
The peak sample selection module 326 can include an
analog-to-digital converter 328 coupled to receive the PDAC signal
322a and configured to generate digital samples 328a of the PDAC
signal 322a.
The peak sample selection module 326 can also include a digital
memory 330 such as in the form of a shift register that can be
sized to hold N samples associated with N cycles of the DIFF signal
306a. In one embodiment, the digital memory 330 is configured to
store at least four samples of the PDAC signal 322a associated with
four cycles of the DIFF signal; namely a sample from the present
cycle and samples from each of the three cycles immediately
preceding the present cycle. It will be appreciated by those of
ordinary skill in the art that samples from not only one or more
prior revolutions and from one or more target features from such
revolutions can be stored, but also the number of samples stored
can be varied.
However, as noted with respect to FIGS. 3 and 3A, various
combinations of peak signal samples can be used to generate the
thresholds and thus, various combinations of PDAC signal samples
may be stored in the memory 330. For example, the memory 330 can
store samples of the PDAC signal 322a associated with M revolutions
of the target 24, each one of the revolutions associated with N
samples. Thus, in some embodiments, the memory 330 can be sized to
hold M.times.N multi-bit samples of the PDAC signal 322a. In some
embodiments, the digital memory 330 can store samples of the PDAC
signal 322 not associated with every target features 24a-24c (i.e.,
every DIFF signal cycle), but associated with only some of the
target features. In other embodiments, the digital memory 330 can
store samples of the PDAC signal 322a not associated with every
target revolution, but associated with only some revolutions. In
general, the stored samples can be from all or some previous
revolutions from which revolutions the stored samples can be from
all or some target features from those revolutions. These
arrangements can use a reduced amount of digital memory 330 and a
reduced amount of circuit die area. Another use for retaining the
history of PDAC signal in the memory 330 is that, if a peak
associated with a particular target feature (i.e., gear tooth)
deviates greatly from revolution to revolution, the deviation can
be used to indicate a fault in the magnetic field sensor 300.
The peak sample selection module 326 can also include a function
processor 336 coupled to the digital memory 330 and configured to
process a plurality of selected samples 332a selected from among
the stored samples 332. The function processor 336 can be an
averaging circuit responsive to a control signal 334a for selecting
x particular samples 332a to be averaged in order to provide an
averaged signal 336a. Each one of the x sample words is clocked to
a new sample word in accordance with transitions of the PosComp
signal 308a and thus, each of the x sample words is actually a
stream of sample words, each representative of a particular cycle
of the DIFF signal 306a at or prior to a present cycle of the DIFF
signal 306a. Thus, when referring to a sample, it will be
understood that the sample is actually a stream of samples. The
signal 336a provides a stream of samples, each one of which is an
average of a set of x samples 332a.
In other embodiments, the signal 336a is an RMS average of each set
of x samples 332a. In other embodiments, the signal 336a is a
weighted average of the each set of x samples 332a, for example,
taking more recent samples with a higher weight than earlier
samples. It will be appreciated that other combinations of samples
may be achieved with the function processor 336.
The peak sample selection module 326 can also include a multiplexer
338 configured to select any number of the stored samples 332 of
the PDAC signal 322a, each selected sample 332b associated with a
different cycle of the DIFF signal 306a. The multiplexer 338 can
select the y sample words 332b in response to a control signal
334b.
The multiplexer 338 is configured to provide either the y selected
samples 332b (which can be one or more samples) to a DAC 340 or the
averaged samples 336a to the DAC 340 under the control of a control
signal 334b. More particularly, a control circuit 346 provides the
control signals 334a and 334b to control how many (x) and which of
the stored samples 332 are processed by the function processor 336,
how many (y) and which of the stored sample(s) 332b are coupled
directly to the multiplexer 338, and which of the streams 336a,
332b provide the multiplexer output signal 338a coupled to the DAC
340. For example, for a time shortly after the circuit 300 is first
powered on, e.g., during a calibration time period, the multiplexer
338 can select as its output signal 338a, the peak signal sample
332b, and thereafter, e.g., during a running mode of operation, the
multiplexer 338 can select as the output signal 338a, the averaged
signal 336a. More particularly, during certain times of operation,
the peak sample selection module 326 selects one or more samples
332b from the memory 330 on each cycle of the DIFF signal, each
respective sample corresponding to the present cycle of the DIFF
signal 306a and during other times of operation, the peak sample
selection module 326 processes, e.g., averages, selected samples
332a, for example, samples from the present and three prior cycles
of the DIFF signal 306a to generate the selected peak signal 326a.
This arrangement of selecting samples 332b for passing through the
multiplexer without processing during the calibration mode is
advantageous, since immediately after power up, there is no
threshold history, and peaks from previous cycles are not available
and/or are not accurate.
The DAC converter 340 is coupled to receive the signal 338a and
configured to generate an analog sample, or more precisely, a
series of analog samples 326a (referred to herein as the positive
selected peak signal 326a), according to a series of digital
samples 338a. It will be appreciated that a filter (not shown) can
be used to smooth the selected peak signal 326a. It will also be
appreciated that the peak signal samples may be stored in the
register 330 in analog form, in which case the analog-to-digital
converter 328 may be omitted and the memory 330 may be replaced by
a bucket brigade device (BBD) or the like which will be understood
to be an analog shift register capable of storing and shifting
discrete analog samples. In this case, an analog function circuit
configured to perform a function upon stored analog signal samples
may replace the digital function processor 336. The function
performed by the analog function module can be the same as or
similar to the functions described above in conjunction with the
averager of FIG. 5.
Generation of the threshold signal 316 in accordance with a
function of a plurality of peak signal samples from prior cycles
establishes a more accurate threshold signal 316 and a resulting
motion signal 308a that is less susceptible to mechanical
irregularities, wobble, or runout of the target 24 or other sensor
system disturbances.
Referring now to FIG. 6, in which like elements of FIG. 5 are shown
having like reference designations, a circuit 450 can have
characteristics similar to those of the circuit 300 of FIG. 5;
however, some of the analog circuits shown in FIG. 5 are replaced
by corresponding digital circuits. For example, the peak identifier
322 of FIG. 5 can be replaced by a peak identifier 456 that
implements the peak identifier functionality of tracking and
holding positive and negative peaks of the DIFF signal with a logic
circuit 458. The peak identifier logic circuit 458 is coupled to
receive a version of the DIFF signal 306a that has been digitized
by an analog-to-digital converter 452, and is configured to
generate a positive peak tracking PDAC signal 456a and a negative
peak tracking NDAC signal 456b, both of which are digital
signals.
A peak sample selection module 426 can be entirely digital, not
requiring the analog-to-digital converter 328 or the DAC 340 of
FIG. 5. The peak sample selection module 426 is configured to
generate a positive selected peak signal 426a, similar to the
positive selected peak signal 326a of FIG. 5, but which can be a
digital signal.
In the embodiment of FIG. 6, a negative peak sample selection
module is omitted and the NDAC signal 456b is coupled directly to
the threshold generator 442. It will be appreciated that this
approach of using a positive selected peak signal 426a and the NDAC
signal 456b to generate the threshold 464a may be sufficient to
gain at least some of the threshold accuracy benefits of the
invention. For example, in some magnetic field sensing element
configurations such as those in which a single element detects the
target features (e.g., FIG. 1), the positive DIFF signal peaks may
vary more significantly and more quickly in response to target
anomalies than the negative DIFF signal peaks. Thus, in such
embodiments, a negative peak sample selection module may be
omitted. Whereas in other magnetic field sensing element
configurations, such as those in which multiple elements are used
to generate the DIFF signals as the difference between signals from
multiple magnetic field sensing elements (e.g., FIG. 1A), the
positive and negative peaks of the DIFF signals generally vary to
the same extent and in the same manner in response to target
anomalies. Thus, in these types of embodiments, it is generally
desirable to include both the positive and negative peak sample
selection modules.
The circuit 450 can include a digital threshold generator 442
including a logic circuit that, like the analog threshold generator
380 of FIG. 5, can generate a threshold signal 464a that is at a
first level corresponding to a first percentage of the difference
between the positive selected peak signal 426a and the NDAC voltage
456b when the DIFF_Dig signal 452a exceeds the threshold signal
464a and is at a second level corresponding to a second percentage
of the difference between the positive selected peak signal 426a
and the NDAC voltage 456b when the DIFF_Dig signal is less than the
threshold signal 464a.
The circuit 450 can include a digital comparator 454 coupled to
receive an output signal 464a from the threshold generator 442 and
also coupled to receive the digitized DIFF_Dig signal 452a. The
digital comparator 454 is configured to generate a PosComp signal
454a, which can be the same as or similar to the PosComp signal
308a of FIG. 5.
It will be appreciated that many of the functions of the circuit
450 are implemented with digital circuits that perform the same or
similar functions to the analog circuits of the circuit 300 of FIG.
5.
Referring now to FIG. 7, an alternative threshold generator 460 is
shown that may be used in place of the threshold generators of
FIGS. 5 and 6. It will be appreciated that in the context of the
digital circuitry of FIG. 6, the selected peak signal 426a and NDAC
signal 456b would require conversion to an analog form for use of
the threshold generator 460. Threshold generator 460 may include a
resistor ladder 470 across which the positive selected peak signal,
such as 326a of FIG. 5 or signal 426a of FIG. 6, and the negative
selected peak signal 344a of FIG. 5 or the NDAC signal 456b of FIG.
6 are coupled.
The resistor ladder 470 may have a center tap 470 at which the 50%
point between the voltage across the resistor ladder is provided.
The center tap 470a may be coupled to offset voltage sources 466
and 468 via switches 462 and 464, respectively, as shown. The
switch 461 may be controlled by the PosComp signal, such as signal
454a of FIG. 6, and the switch 464 may be controlled by an inverted
version, N-PosComp, of the PosComp signal. With this arrangement,
the threshold signal 460a, which may be the same as or similar to
the threshold signal 316 of FIG. 5 or threshold signal 464a of FIG.
6, may be provided as a predetermined offset voltage (established
by the voltage source 466) greater than the mid-point between the
positive selected peak signal and negative selected peak signal (or
NDAC signal) when the DIFF signal is less than the threshold and as
a predetermined offset voltage (established by the voltage source
468) less than the mid-point between the positive selected peak
signal and negative selected peak signal (or NDAC signal) when the
DIFF signal is greater than the threshold.
Referring now to FIG. 8, another alternative threshold generator
480 suitable for use in place of the threshold generators of FIGS.
5 and 6 is shown to include an offset voltage source 486 having a
first offset voltage source 486a for coupling to the positive
selected peak signal 326a of FIG. 5 or 426a of FIG. 6 and a second
offset voltage source 486b for coupling to the negative selected
peak signal 344a of FIG. 5 or the NDAC signal 456b of FIG. 6. It
will be appreciated that in the context of the digital circuitry of
FIG. 6, the selected peak signal 426a and NDAC signal 456b would
require conversion to an analog form for use of the threshold
generator 480.
The offset voltage source 486a is coupled to a switch 482 that is
controlled by the PosComp signal and the voltage source 486b is
coupled to a switch 484 that is controlled by an inverted version,
N-PosComp, of the PosComp signal. With this arrangement, the
threshold signal 464a, which may be the same as or similar to the
threshold signal 316 of FIG. 5 or threshold signal 464a of FIG. 6,
may be provided as a predetermined offset voltage (established by
the voltage source 486a) less than the positive selected peak
signal when the DIFF signal is less than the threshold and as a
predetermined offset voltage (established by the voltage source
486b) greater than the negative selected peak signal (or NDAC
signal) when the DIFF signal is greater than the threshold.
Referring now to FIG. 9, a method 500 for computing the threshold
signals, such as signals 216a-216h of FIG. 3, begins with an
initialization step 504 during which the circuitry is reset to a
known state. In step 508, the present peak values of the DIFF
signal as tracked by the peak identifier, such as peak identifier
322 of FIG. 5 or peak identifier 456 of FIG. 6, are adjusted, or
updated. Various schemes are possible for updating the PDAC and
NDAC signal levels, some of which are described in the
above-referenced U.S. Pat. No. 6,525,531 and others of which are
described in U.S. Patent Application Publication Nos. 2011/0298447,
2011/0298448, and 2011/0298449, which applications are assigned to
the Assignee of the subject invention and are hereby incorporated
by reference. For example, the update logic of peak identifier 322
of FIG. 5 or logic 458 of peak identifier 456 of FIG. 6 may be
configured to allow the PDAC and NDAC voltages to track the DIFF
signal outwardly (i.e., the PDAC voltage follows the DIFF signal to
its positive peaks and the NDAC voltage follows the DIFF signal to
its negative peaks), but upon each transition of the PosComp
signal, the PDAC and NDAC voltages selectively allowed to move
"inwardly" (i.e., PDAC decreases and NDAC increases) to the level
of the DIFF signal.
In a step 512, it is determined whether automatic gain adjustment
(AGC) and/or automatic offset adjustment (AOA) functions should be
performed, by determining whether the DIFF_Dig signal (or the DIFF
signal depending on the embodiment) has passed a learning rail
level that is associated with predetermined DIFF signal gain and
offset conditions. If it is determined that the DIFF_Dig signal has
passed the learning rail, then in step 516, the DIFF_Dig signal
offset and/or gain are adjusted, such as with AGC and AOA circuitry
and techniques described above. In step 520, a peak count value
(e.g., a counter in the control circuit 346 of FIG. 5 or control
circuit 448 of FIG. 6) is reset.
Once the DIFF_Dig signal has been gain and/or offset adjusted if
necessary, the thresholds are computed starting in step 524 in
which it is determined whether a predetermined number of DIFF_Dig
signal peaks has occurred, such as four peaks in one embodiment.
This step can be achieved with a counter in the control circuit 346
of FIG. 5 or 448 of FIG. 6 for example. If the predetermined number
of peaks has not occurred, then in step 528, thresholds are
computed based on the peaks in the present DIFF signal cycle, as
demonstrated in the shown example, or using a predetermined simpler
threshold such as a fixed threshold. For example, in each of the
first three DIFF signal cycles in FIG. 3, the thresholds 216a-216f
are based on the peaks detected in the respective cycle, as may be
achieved by the multiplexer 338 (FIG. 5) passing through the stored
peak 332b from the current cycle. Recall that this manner of
computing the thresholds is advantageous during an initial,
sometimes referred to as a calibration, time period after power up
when there is insufficient prior peak history.
If however it is determined that the predetermined number of peaks
has occurred, then in step 532, the thresholds are alternatively
computed as a function of a predetermined set of stored peak signal
samples. For example, upon detecting that four peaks of the DIFF
signal have occurred, the thresholds 216g-216h (FIG. 3) may be
computed as an average of the peak sample from the current cycle
and the peak samples from each of the three prior cycles, as may be
achieved by the averager 336 of FIG. 5 for example.
It will be appreciated that alternatively, the average of four
samples of the peak signal may be computed from four samples of the
peak signal taken during four prior cycles of the DIFF signal
(rather than from three samples taken during three prior cycles and
a sample taken during the present cycle). It will also be
appreciated that while four peak signal samples is discussed in the
illustrative embodiment herein for use in generating the
thresholds, other numbers of peak signal samples from prior DIFF
signal cycles or from prior and present DIFF signal cycles is
possible. Use of digital electronics can be conducive to averaging
a number of samples that is a power of two (e.g., 2, 4, 8, 16,
etc), although other numbers of samples can also be used. The
number of averaged samples is generally based on weighing the
threshold accuracy benefits against the additional implementation
"cost" of processing more samples. The use of four peak signal
samples has been found to achieve a worthwhile threshold accuracy
improvement over the use of two signal samples and eight signal
samples was not found to achieve a significant enough advantage
over the use of four signal samples. Occurrence of the
predetermined number of peaks can end the calibration mode of
operation and begin the running mode of operation in step 532.
Once the thresholds are computed in step 528 or 532, the state of
the PosComp signal is determined in step 536. If the state of the
PosComp signal is a logic zero, it is determined in step 540 if the
DIFF_Dig signal is greater than the operate threshold Bop (e.g.,
such as threshold 216g of FIG. 3), and if it is, then the PosComp
signal level is changed to a logic one in step 548, the PDAC signal
is sampled and stored in memory in step 552, and a peak count value
maintained in the control circuit, such as control circuit 346 of
FIG. 3 or control circuit 448 of FIG. 6, is incremented.
Alternatively, if the PosComp signal level is at a logic one, then
it is determined in step 544 whether the DIFF_Dig signal is less
than the release threshold Brp (e.g., such as threshold 216h of
FIG. 3) in step 544, and if it is, then the PosComp signal level is
changed to a logic zero in step 560, the NDAC signal is sampled and
stored in memory in step 564, and the peak count value is
incremented. In other embodiments, it is also possible to store
both NDAC and PDAC values at every change of PosComp.
In step 572, a system monitoring function of the control circuit is
checked and if the system monitor check is positive, then the
process repeats with the peak value adjustment step 508.
Alternatively, if the system monitor check is negative, then the
PDAC and NDAC (e.g., the PDAC and NDAC in the peak identifier 322
of FIG. 5) and the peak count value (e.g., the counters in the peak
identifier 322 of FIG. 5) are reset in step 576 following which the
process repeats with the peak value adjustment step 508, as shown.
The system monitoring function of the control circuit can monitor
various system functions. As one example, an additional set of
thresholds is compared to the DIFF signal and if several
transitions occur based on the additional thresholds when no
transitions occurred based on the above-described thresholds, then
a fault indication is provided.
It should be appreciated that parts of the circuits of FIGS. 5-8
can be interchanged with each other. For example, an analog peak
sample selection module such as the peak sample selection module
326 of FIG. 5 can be used in the circuit of FIG. 6.
All references cited herein are hereby incorporated herein by
reference in their entirety.
Having described preferred embodiments of the invention, it will
now become apparent to one of ordinary skill in the art that other
embodiments incorporating their concepts may be used. It is felt
therefore that these embodiments should not be limited to disclosed
embodiments, but rather should be limited only by the spirit and
scope of the appended claims.
* * * * *